This invention relates to a pneumatic tire having an apex in the region of the steel cord reinforced carcass ply turn-up. More specifically, the invention relates to such a tire having an apex comprised of a diene rubber and a trans 1,4-polybutadiene resin.
The term xe2x80x9capexxe2x80x9d as used herein refers to the area of the tire in the immediate proximity of the carcass ply turn-up. The apex includes a rubber wedge located in the lower sidewall region above the bead and is bonded to and encased by the carcass plies. The apex also includes the area located between the lower sidewall rubber and the axially outer side of the carcass ply turn-up.
A tire is a composite of several components each serving a specific and unique function yet all synergistically functioning to produce the desired performance. One important component is the carcass ply. The carcass ply is a continuous layer of rubber-coated parallel cords which extends from bead to bead and functions as a reinforcing element of the tire. The ply is turned-up around the bead, thereby locking the bead into the assembly or carcass. The tire is assembled in the green (uncured) state and upon completion is then vulcanized. Unfortunately, prior to vulcanization when steel cord is the reinforcement in the carcass ply, the carcass ply turn-up can cause deformation of the components which upon vulcanization results in an unacceptable product. The deformation of the steel cord reinforced carcass ply turn-up in the area of the apex is known as xe2x80x9capex creepxe2x80x9d. Apex creep is a distortion of the apex compound at the steel cord reinforced carcass ply turn-up endings without an interfacial separation due to the stresses associated with the steel cord reinforcement in the carcass ply. Conventionally, natural rubber is used as the rubber in the apex area along with additional reinforcement provided by nylon or Flexten(copyright) chippers.
The use of trans 1,4-polybutadiene has been disclosed for various purposes, including, for example, tire tread rubber compounds and increasing green strength of rubber mixtures (see Japanese Patent Publication Nos. 60-133,036; 62-101,504 and 61-143,453) and U.S. Pat. No. 4,510,291.
Uniquely, trans 1,4-polybutadiene is typically a thermoplastic resin rather than rubber in its uncured state at room temperature by virtue of its high crystallinity. Because it contains many double bonds in its backbone, it can, however, be suitably blended and co-cured with elastomers.